1. Field of Invention
This invention relates to improvements in automotive clutches and, in particular, to a clutch with improved frictional facings.
2. Brief Statement of the Prior Art
In most automotive and truck vehicles, torque is transmitted to the drive shaft through a clutch disc which is mounted on the transmission input shaft and which is received between a pressure plate of the clutch and the flywheel. Resilient springs, either coil springs or a Bellville spring, apply a resilient force to the pressure plate to bias it towards the flywheel, frictionally securing the clutch disc between the pressure plate and the flywheel, and a lever mechanism is provided to release the spring force and retract the pressure plate to disengage the clutch.
Asbestos facings have been used for many years and have served suitably as frictional facings on the surfaces of the clutch disc. These facings however, have fallen into disfavor because of the health hazard presented by asbestos, and recent attention has focused on alternative materials. Of these alternative facings, the most commonly used have been organic, and/or composite, facings. The composite facings are formed of carbon or various resins which are reinforced with non-toxic fibrous materials, and are commonly referred to as organic composites, although in some instances they are formed of carbon rather than organic materials. As used hereinafter, these will be referred to as organic composite facings and this term is intended to include all carbon and organic facings.
A difficulty with the organic composite facings is that they are entirely unsuited for use at high temperatures. The coefficient of friction of the organic composite materials declines dramatically at elevated temperatures, typically at temperatures in excess of 500 degrees F. These temperatures can be quickly achieved on the surfaces of the frictional facings of a clutch, particularly if there is an excessive amount of sliding movement of the members during clutch engagement. This results in an accelerated failure since as the temperature increases, the coefficient of friction decreases, causing more slippage and frictional rubbing of the surfaces, which further increases the temperature, resulting in premature wear and destruction of the surface. Often the organic composite facings are damaged by a permanent surface glazing, which results from overheating of the clutch. When this occurs, the facing looses its original frictional properties and significantly deteriorates in performance.
In severe, high usage commercial applications, sintered metal facings are commonly used. These sintered metal facings are frequently provided as a friction facing in the form of a disc, or individual pucks, which are generally trapezoidal shaped members that are bonded or secured with fasteners to the surfaces of the clutch disc, pressure plate, or flywheel. The sintered metal facings have very low coefficients of friction at low and ambient temperatures. At temperatures in excess of several hundred degrees F., however, the sintered metal facings exhibit very acceptable coefficients of friction.
Attempts have been made to adapt the sintered metal facings to clutch members for normal automotive applications, however, these attempts have not been successful, primarily because of their abrupt, uncontrollable characteristics and because they are two to three times as heavy as organic composite facings. Their greater weight creates excess inertia loading resulting in excess wear of transmission components. In fact, one noted automotive authority, Mr. Thomas Monroe, has written that sintered metal facings are entirely unsuited for normal automotive use and will never be used in normal applications, Clutch and Flywheel Handbook p 66 (1987). The greater weight of sintered metal facings and their low coefficient of friction values at normal operating temperatures results in vibration, slipping, chattering and premature wear of the clutch.